Chp 8 Cell Division
Sequences of images illustrating the process of nuclear division, mitosis
Cell Division Plays Many Roles
✶ Organisms
reproduce by two
methods
– Asexual reproduction
• Offspring identical to parent
• Inheritance of all genes from
one parent
– Cell dividing by mitosis
Sexual reproduction
• Offspring similar to parents, but
show variation in traits
• Inheritance of unique sets of
genes from two parents
Human family showing offspring
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from sexual reproduction
2 Types of Cell Division Occur in Our Life Cycle
A fertilized egg, or zygote, divides by
mitosis. The fetus grows into a mature
adult consisting of countless cells with
identical DNA.
Each mature individual
produces sex cells by another
form of cell division called
meiosis.
Sex cells combine at
fertilization.
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Clicker Question
Shortly after fertilization, a zygote divides
into two identical cells. What type of cell
division produces these two cells?
A. mitotic cell division
B. meiotic cell division
C. apoptosis
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Prokaryotes Divide by Binary Fission
✶ Binary
fission = “dividing in half”
– An asexual process
– Two identical cells arise from one cell
– Stages in the process
1. Single chromosome duplicates, and the copies begin to
separate from each other
2. The cell elongates, and the chromosome copies
separate further
3. The plasma membrane grows inward at the midpoint to
divide the cell into two daughter cells
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Plasma
membrane
Prokaryotic
chromosome
Cell wall
Binary fission
of a prokaryotic
cell
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1
Duplication of chromosome
and separation of copies
2
Continued elongation of the
cell and movement of copies
Division into
two daughter cells
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Clicker Question
Why is binary fission an asexual
process?
A. It occurs only in prokaryotes.
B. It does not require DNA replication.
C. It produces two identical cells.
D. The chromosome is circular.
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DNA Replication Precedes Cell Division
✶ Eukaryotic
chromosomes are made of chromatin
– DNA + proteins in a loose, uncoiled form
– To prepare for division, chromatin becomes coiled and
chromosomes become visible (using LM)
– Well before division, DNA replicates forming 2 copies
• Duplicated chromosome has two sister chromatids,
(identical DNA molecules)
• Sister chromatids joined at centromere, a narrow
“waist”
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✶ Before
cell divides, all
chromosomes replicate
– Replication produces sister
_____________
Chromosome
duplication
Sister
chromatids
Centromere
✶ During
division, sister
chromatids separate
– each daughter cell gets a
complete and identical set of
chromosomes
Chromosome
distribution
to
daughter
cells
Chromosome duplication and distribution
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Chromosomes
DNA molecules
Sister
chromatids
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to the
daughter
cells
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The Cell Cycle Multiplies Cells
✶ The
cell cycle - an ordered sequence of events in
a cell’s life
✶ 2
stages
– Interphase: duplication of cell contents
• G1—growth, increase in cytoplasm
• S—duplication of chromosomes
• G2—growth, preparation for division
– Mitotic Phase: division of cell
• Mitosis - division of the nucleus
• Cytokinesis - division of the cytoplasm
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Mitotic Cell Division Generates Exact Copies
The cell cycle describes
the events that occur in one
round of cell division.
Mitosis is the division of
the nucleus, and
cytokinesis is the
division of the cell itself.
Figure 8.8
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Cell Division is a Continuum of Changes
✶ Mitosis
– nuclear division forming 2 nuclei
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
✶ Cytokinesis
– cytoplasmic division forming 2 cells
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✶ A
mitotic spindle guides the separation of the 2
sets of daughter chromosomes
– Composed of microtubules- straight hollow tubes of
proteins called tubulin
– Produced by centrosomes, football-shaped structures
in the cytoplasm (in animals contain centrioles)
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Chromosomes Divide During Mitosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Figure 8.9
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Clicker Question
In some fungi and slime molds, mitosis
may occur without cytokinesis. What
would you expect to find in these
species?
A. Cells that don’t contain nuclei.
B. Cells that contain multiple nuclei.
C. DNA that never condenses into visible
chromosomes.
D. Nuclei that never enter interphase.
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Review: Mitosis Provides for Growth, Cell
Replacement, and Asexual Reproduction
Functions of Mitosis
✶ Growth
– New cells produced
✶ Cell
– Replacement
Lost or damaged cells
replaced
✶ Asexual
– Onion root cells
Reproduction
New organism formed (Hydra)
Hydra
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Cancer Arises When Cells Divide Out of Control
✶ Chemical
checkpoints
regulate the cell
cycle
✶ But what if the
body loses control
of cell division?
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✶ A tumor is an
abnormal mass of
tissue
✶ Tumor cells
divide out of
control
✶ Tumors are
either benign or
malignant
Figure 8.11
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Genes and the environment
both can increase cancer risk
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Cancer treatments include
surgery, drugs
(chemotherapy), and
radiation
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Mastering Concepts
What keeps cells from dividing when
they are not supposed to?
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Investigating Life: Cutting Off a Tumor’s
Supply Lines in the War Against Cancer
✶ Tumor cells mutate
and divide quickly,
often developing
resistance to drugs.
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Cutting Off a Tumor’s Supply Lines
✶ One class of drugs
does not target
cancer cells
✶ Instead, the drugs
attack the blood
vessels that supply
nutrients to the tumor
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Cutting Off a Tumor’s Supply Lines
✶ Cells
lining blood
vessels divide and
accumulate mutations
slowly
✶ Therefore,
blood
vessels are less likely
than tumor cells to
become resistant to
drugs
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Cutting Off a Tumor’s Supply Lines
✶ Endostatin targets
blood vessel cells. It
reduced
tumor growth in
mice.
✶ Traditional
chemotherapy drugs
delay cancer growth,
but do not prevent it.
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Mastering Concepts
Why are traditional chemotherapy
drugs often most effective during
the first few treatments?
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